Memories of very apparent, highly visible pollution occur in scientists’ accounts of their childhood: blackened buildings in northern cities and dense smogs in London. With the Clean Air Act (1956) many looked forward to a less polluted future, but through the second half of the twentieth century scientists have detected and measured the effects of a large number of less visible pollutants. Detection has taken many forms: an impression that summer air in the countryside was becoming hazy (CFCs), strange flecks on tobacco leaves (ozone), silent springs (pesticides), crumbling cathedrals (acid rain), yellowing ryegrass (SO2), measurements of lead in Antarctic snow, a hole in the ozone layer (CFCs), fishless lakes in Scandinavia and Scotland (acid rain), algal blooms (eutrophication). Of all the invisible pollutants, perhaps the most unsettling has beenradioactivity – as clouds of fallout from accidents such as Windscale (1957) and Chernobyl (1986) and as containers of waste to be disposed of. In defending measurements, advising governments and speaking to the media about pollution, scientists have often found themselves out of alignment with the interests of industry and commerce on the one hand, and a growing ‘environmental movement’ on the other.